The present invention relates to a digitally implemented method of short term forecasting of moderate size reservoir-triggered earthquakes. More particularly the present invention relates to short-term earthquake forecasting method for exactly foreseeing the hypocenter and the date of the occurrence of a moderate size reservoir-triggered earthquake.
The reservoir-triggered earthquakes can lead to significant damages because of their location in the vicinity of thickly populated towns (Gupta and Rastogi, 1976). Presently, there are about 70 cases of reservoir-triggered seismicity in the Globe (Gupta, 1992). Damaging earthquakes exceeding magnitude 6 occurred near large reservoirs at Hsinfengkiang in China, at Kariba in the Zambia-Zimbabwe border region, at Kremasta in Greece, and at Koyna in India. So far, the largest and most damaging reservoir triggered earthquake occurred on Dec. 10, 1967 at Koyna region which claimed over 200 human lives, 1500 injured and rendered thousands homeless (Gupta et al., 1972, 1976, 1983, 1999 and 2002). Till date, one M6.3, seventeen earthquakes of Mxe2x89xa75 (where M is the intensity), about 170 earthquakes of Wxe2x89xa74 and hundred thousand of smaller events. Earthquakes of Mxe2x89xa75 occurred in 1967, 1968(probably an aftershock of 1967 event), 1973, 1980, 1993-94 and 2000 when certain conditions of reservoirs filling parameters are met (i.e. water level in Koyna and/or Warna reservoir exceeding the previous maximum (Kaiser effect), rate of loading exceeding 12 m/week and the retention time of high water level (Talwani, 1997; Rastogi et al., 1997; Gupta et al., 2002). It is inferred based on the estimation of moment release by Mxe2x89xa75 (where M is the intensity of earthquake) earthquakes including one M6.3 that the activity will continue for 34 decades with Mxe2x89xa75 earthquakes (Gupta et al., 2002). Thus, short-term prediction of Mxe2x89xa75 earthquakes will be quite important to mitigate seismic hazard of the region. It will also be important method for earthquake prediction for several other reservoir-triggered sites of the world. Earthquake prediction is debatable (Mogi, 1969). Nevertheless, there are at least three types of (conventional) earthquake prediction. Deterministic prediction is where the behavior before the earthquake (the stress interactions with the surrounding rocks, say) can be calculated (by whatever techniques are available) so that the time, place, and magnitude of the future large earthquakes can be estimated within well-defined windows (Di Luccio et al., 1997). Earth is complex, non-linear and heterogeneous at all scales which makes deterministic prediction difficult. Statistical prediction is where seismicity in the past can yield estimates of seismicity in the future. Statistical analysis of seismicity in the past in order to attempt to predict future behavior again fails because of complexity and heterogeneity. The third and most common type is where some key precursory phenomenon or a group of phenomena indicate that a large earthquake is imminent (Agnew and Jones, 1991). I suggest that all three types cannot predict time, place, and magnitude of a future large earthquake. It is complexity and heterogeneity that prevents it each time.
Foreshocks are just the small fraction of mainshocks that trigger aftershocks at the high end of the Gutenberg-Richter magnitude distribution, and thus with magnitudes greater than themselves (Utsu, 1969; Ogata et al., 1995; Abercrombie and Mori, 1996). Immediate foreshocks are the only incontrovertible, causal earthquake precursor but their cause and their relationship to their mainshock is not obvious (Dodge and Beroza, 1995; Many studies have demonstrated the strongly non-random clustering of foreshocks with mainshocks (e.g., Papazachos, 1975; Bowman and Kisslinger, 1984; Ohnaka, 1992; Console et al., 1993; Savage and dePolo, 1993; Ogata et al., 1995), but without a consistent idea emerging of the physical relationship between foreshocks and mainshocks. Foreshocks have been considered as accelerating failure (e.g., Jones and Molnar, 1979), as triggers of the mainshock (e.g., Dodge et al., 1995) and as mechanisms to delay the mainshock (e.g., Jones et al., 1982; Jones, 1984; Dodge et al., 1997). Earthquakes come in a range of magnitude 1 to 9, and they exhibit clustering in both space and time. Power law distributions of number versus energy and energy versus time before and after large events suggest that a complex systems approach to earthquake mechanics may yield new insights into the spatio-temporal distribution of slip for individual earthquakes, and into the spatio-temporal patterns of regional seismicity.
Foreshocks have been used to detect the nucleation process of large- or moderate size earthquakes, which in turn, leads to earthquake prediction. Quasi-static slip within the nucleation zone preceding the large/moderate size earthquakes has been noticed. For 1978 Izu peninsula earthquake of M7.2, the nucleation zone grew at a rate of 1 to 40 cm/sec before reaching a diameter of 10 km within one day of the mainshock (Ohnaka, 1992). According to this theory, foreshocks will cluster before the occurrence of mainshocks, therefore, accurate estimation of hypocentral parameters of foreshocks will make possible to detect foreshock clustering. The nucleation zone deepens with depth if mainshocks are occurring within the seismogenic layer otherwise nucleation zone propagates upwards. Accordingly, this theory obviously means that all of the earthquakes should nucleate at a point where the stress level exceeds critical level and then the rupture propagates along the fault zone causing foreshocks at asperities (Rastogi and Mandal, 1999; Singh et al., 1998; Mandal et al., 2000). And finally the nucleation zone reaches the base of the seismogenic layer to cause the main shocks.
The above-mentioned theory involves an assumption that the foreshocks should occur in a cluster and mainshock should occur within the seismogenic layer, seems to be rather unsuitable to predict earthquake, which occurred below the seismogenic layer, which may nevertheless cause a considerable damage. Thus, this method seems to be suitable for reservoir-triggered earthquakes, which generally would occur in a cluster due to the influence of fluids at hypocentral depth, characterized by shallow focal depths.
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The primary object of the present invention is to provide an earthquake forecasting method capable of foreseeing occurrence of a future earthquake and the epicenter in a simple manner.
Another object of the present invention is to obtain accurate and reliable hypocentral parameters (ERH less than 0.5 km and ERZ less than 1 km and RMS less than 0.1 s) in real time, which needs expensive telemetered network of digital stations.
Yet another object of the present invention is to detect foreshock clustering in real time. Further object of the present invention is to detect the nucleation zone for studying deepening of nucleation zone with time in real time.
Yet another object of the present invention is to make a successful short-term prediction of moderate size Koyna-Warna events.
The present invention provides a new earthquake forecasting method which comprises combined steps of observing/detecting a foreshock clustering/nucleation zone at shallow depth over a 100 hours period prior to the mainshock occurrence and observing/studying the deepening of nucleation zone with time to forecast, with a considerable accuracy, a future moderate size reservoir-triggered earthquake and the epicenter which may occur within two days after the foreshock clustering is observed.